Schottky Power Diodes are also Zeners in the 20 to 40V range at high currents and may be chosen to handle this for some rated Watt and Joule values.

Although the energy stored in current in the Inductor is released into the Zener E= ½ LI² , I(t) is a ramp and not constant.  

> But what is the Power Dissipation? 

Yet we know when the transistor is switched off, Iz=IL and for now, if we let Vce(sat)=0, the voltage across the inductor = 5V , thus the final power input while the ramp reaches a peak current , Ip  is **P = ½ VIp**  ( triangle is ½ the area of the VI product square)
 

The Zener will discharge the current faster but with a fast step  voltage and some triangular ramp decay time. 

  V=LdI/dt and both L and dI=Ip are constants at time T=dt of current ramp up  for the ramp-up time, T. 
 
The discharge time duration **Td = Vcc/Vz * T**  since we already concluded Vdt = LIp = some fixed value.

Although the semiconductor doping of Avalanche diodes is lighter, has a  higher breakdown voltage, is faster and has a different conduction mechanism, the notation for such diodes often still uses \$i_Z\$.



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  [1]: https://i.sstatic.net/xAS2s.jpg